Focal mechanisms in the northeast New Madrid seismic zone

On 23 January 1812, an estimated M7-M8 earthquake occurred in the New Madrid seismic zone. The New Madrid North fault (NMNF) has been proposed as the location of this rupture. Based on the estimated size of this earthquake, the rupture length of the fault plane would be expected to extend beyond the microseismicity that defines the NMNF. Seismicity data from this area show three alignments of earthquakes, running northeast from New Madrid, Missouri, into western Kentucky and southern Illinois that may represent an extension of the New Madrid seismic zone (NMSZ) and therefore alternative locations for the 23 January 1812 rupture. Earthquakes in the northeast NMSZ from June 1995 to June 2006 were relocated using a velocity model of the Mississippi embayment with appropriate depths to bedrock beneath seismic stations. Focal mechanisms were generated for events on the northeast trending alignments. The results show approximately half the focal mechanisms have a north-northeast-striking nodal plane and a right-lateral, strike-slip component consistent with earlier studies of the NMNF to the southwest. This shows that this part of the northeast NMSZ is influenced by the same fault pattern and stress regime as the NMNF, may be an extension of the NMSZ, and therefore alternate locations of the 23 January 1812 rupture.